In conventional wireless communications, a single antenna is used at a source to wirelessly transmit a signal to a single antenna at a destination. In some cases, this gives rise to problems due to multipath effects. For example, when a wireless signal (an electromagnetic field) is met with obstructions such as hills, canyons, buildings, and utility wires, wavefronts can become scattered, and thus, may take many paths to reach the destination. Late arrival of scattered portions of the wireless signal can cause problems such as fading, cut-out, and intermittent reception.
Multiple input, multiple output (MIMO) is an antenna technology for wireless communications in which multiple antennas are used at both the source (transmitter) and the destination (receiver). Signals received at the multiple antennas at the source and at the destination are combined to minimize errors and optimize data speed. MIMO is one of several forms of smart antenna technology. Other forms include multiple input, single output (MISO) and single input, multiple output (SIMO), for example. Using smart antenna technology (e.g., multiple antennas at both the source and the destination) can eliminate signal problems caused by multipath wave propagation, and can even take advantage of this effect.
A space-time code (STC) is a method employed to improve reliability of data transmission in wireless communication systems using multiple transmit antennas. STCs rely on wirelessly transmitting multiple, redundant copies of a data stream to a receiver with the hope that at least some of the transmitted data streams may survive a physical path (e.g., wireless communication channel) between transmitting antennas and receiving antennas in a good enough state to allow for reliable decoding. Space-time block codes (STBCs) act on a block of data at once and may provide diversity gain.
MIMO systems that use large dimensional non-orthogonal STBCs may provide advantages over vertical Bell-labs layered space time (V-BLAST) type MIMO systems in that higher spectral efficiency (e.g., information rate that can be transmitted over a given bandwidth in a communication system) as well as full transmit diversity (e.g., transmitting multiple redundant copies of signals) may be provided. In addition to creating dimensionality in space, non-orthogonal STBCs from cyclic division algebras (CDA) can create large dimensionality using a combination of space and time. However, decoding received signals transmitted by such systems using sphere decoding or maximum a posteriori/maximum likelihood (MAP/ML) based methods may be prohibitively expensive.